The quality of video signals can be enhanced by the process of recursive filtering. In this process incoming baseband video signals are proportioned and added to corresponding signals from previous frames of the video signal which are proportioned to supplement incoming video signals. The video signal, to a large degree, is redundant or coherent from frame-to-frame and, thus, signal representing like picture points (pixels) from frame-to-frame sum linearly. Noise components attendant the video signal, on the other hand, tend to be non-coherent and sum as the square root of the sum of the squares of the respective pixel noise components, thus, effecting noise reduction or alternatively signal-to-noise improvement.
In order to perform recursive filtering it is necessary to combine signals corresponding to the same pixels from successive frames. Signal from successive frames may be made available by applying the signal or the signal sums to a delay element having a precise one-frame period delay. The most practical method of developing the frame-delayed signals is to sample the signals and store them in a sampled data delay register, e.g. a charge transfer device, or to convert the sampled signal to digital format and store the digital samples in e.g. a random access memory. Since the tolerance of the frame delay period is extremely critical, the sampling rate is usually selected to be an integer multiple of the frame frequency. The frame delayed signal is accessed by reading the respective samples from memory the same multiple of sample periods after the samples were stored in the memory.
Digital video processing systems may employ several sampling systems. In one such system the sampling clock is locked to the color subcarrier, and in another, the sampling clock is locked to horizontal synchronizing signals. This invention is applicable to the former system and other systems which do not use the techniques of the latter system. It is applicable to video signals in various formats such as NTSC, PAL and SECAM.
Consider, for example a standard NTSC video signal. A sampling clock having a frequency which is an even integer multiple of the color subcarrier frequency will have an integer multiple of clock periods per frame period. This feature facilitates arranging a memory device to produce exactly one frame of delay to applied signals simply by designing the memory device to have the same integer multipl of storage sites which are addressed consecutively.
However, not all TV signals which are compatible for use in NTSC systems conform precisely to the NTSC standard format. For example, signals produced from video recorders and/or video discs have jittering time bases due to instabilities in their transport mechanism. The jitter results in varying frame periods in the reproduced signal. The system processing the signal will develop a sampling signal from the subcarrier contained in the jittering signal so that the number of sample periods per frame period may also vary. In any case, experience has shown that in processing many signals which are nominally NTSC signals, the phase of the sampling clock relative to the horizontal and vertical synchronizing signals varies from frame to frame. This phase variation tends to degrade the performance of a recursive filter due to a reduction in frame-to-frame video signal coherence because a precise one frame period delay is not available as an integer number of clock periods.
In order to fully realize the beneficial potential of a recursive filter, either the input signals or the delayed signals must be time-shifted so that both signals have samples that correspond to the same picture points. This time shifting is denoted "skew correction". Skew as defined herein is the phase difference between the sampling or clock signals and the horizontal synchronizing (Hsync) signal of the incoming signal. Skew is the fraction of a clock period of phase difference between the clock signal and the signal HSYNC. For convenience skew may be measured with respect to the last clock pulse occurring before the center of Hsync and is assumed constant for the duration of each respective line of video signal following an HSYNC pulse. Each pixel of a particular horizontal line has the same skew as the first pixel of that line. Each horizontal line has in general a different skew value except in the case of a standard signal. Thus, a "standard signal" as used herein is a signal in which every horizontal line has the same skew value (which may be zero) and a "non-standard signal" is one in which the skew value varies between one or more lines. A "standard" PAL broadcast signal, because of its 25 Hz offset, is a non-standard signal as herein defined.